Magnetic anisotropy of the van der Waals ferromagnet Cr2Ge2Te6 studied by angular-dependent x-ray magnetic circular dichroism

M Suzuki,A Tanaka,Z Chi,B Gao,A Fujimori,S.-W Cheong,T Koide,K Ikeda,M Kobayashi,Y Takeda,Y Nonaka,G Shibata,S Sakamoto,Y.-X Wan,T Takeda

doi:10.1103/physrevresearch.4.013139

Abstract

The van der Waals ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT) has a two-dimensional crystal structure where each layer is stacked through van der Waals force. We have investigated the nature of the ferromagnetism and the weak perpendicular magnetic anisotropy (PMA) of CGT by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) studies of CGT single crystals. The XMCD spectra at the Cr ${L}_{2,3}$ edge for different magnetic field directions were analyzed on the basis of the cluster-model multiplet calculation. The Cr valence is confirmed to be $3+$ and the orbital magnetic moment is found to be nearly quenched, as expected for the high-spin ${t}_{2g}^{3}$ configuration of the ${\mathrm{Cr}}^{3+}$ ion. A large ($\ensuremath{\sim}0.2$ eV) trigonal crystal-field splitting of the ${t}_{2g}$ level caused by the distortion of the $\mathrm{Cr}{\mathrm{Te}}_{6}$ octahedron has been revealed, while the single-ion anisotropy (SIA) of the Cr atom is found to have a sign opposite to the observed PMA and too weak compared to the reported anisotropy energy. The present result suggests that anisotropic exchange coupling between the Cr atoms through the ligand Te $5p$ orbitals having strong spin-orbit coupling has to be invoked to explain the weak PMA of CGT, as in the case of the strong PMA of ${\mathrm{CrI}}_{3}$.

Highlights

The discovery of graphene has invoked renewed interest in the studies on two-dimensional (2D) materials including layered transition-metal chalcogenides [1,2,3,4]
We have investigated the nature of the ferromagnetism and the weak perpendicular magnetic anisotropy (PMA) of CGT by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) studies of CGT single crystals
The spectra are in good agreement with those reported in the Supplemental Material of Ref. [22] while there are some discrepancies of the X-ray absorption spectroscopy (XAS) spectrum from those reported in Ref. [25], probably due to oxide contributions in the latter data [30,31]

Summary

INTRODUCTION

The discovery of graphene has invoked renewed interest in the studies on two-dimensional (2D) materials including layered transition-metal chalcogenides [1,2,3,4]. Kim et al [22] have performed multisite cluster-model multiplet calculations and attributed the strong PMA of CrI3 to an anisotropic exchange coupling between Cr atoms through the iodine 5p ligand orbitals, which. Cr L2,3-edge XMCD has been reported for CGT [22,25] and CrI3 [26] with a line-shape analysis using atomic-model and cluster-model calculations, and the contribution of the Cr 3d electrons to the PMA has been estimated [22] using the simple model proposed by Bruno [27]. We have carried out Cr L2,3-edge XMCD measurements of a CGT single crystal under magnetic fields applied to different crystallographic directions and subsequent cluster-model multiplet calculations to directly estimate the SIA energy of the Cr atom centered in the CrTe6 octahedron

METHODS

RESULTS AND DISCUSSION

Spin and orbital magnetic moments

Crystal-field splitting

Magnetic anisotropy energies

CONCLUSION